The present invention relates to optical devices, and more particularly, to a sealed prism assembly.
In a recently developed surveying device a pulse of coherent light is beamed from one point toward a reflector located at a second point. The distance between the two points is determined from the round-trip time of travel of the light pulse. The reflector in the device is a trihedral prism constructed to direct reflected light along the path of the incident light, independent of the angle of light incidence. This allows light aimed at the prism to be reflected directly back toward the apparatus, without adjusting the angular disposition of the prism with respect to the incident light.
One problem associated with trihedral prism reflectors, and other similar optical devices, known in the prior art is accumulation of dust and other air-borne material on the inner faces of the prism. Particle accumulation on the prism faces reduces the reflectivity of the prism, thus reducing the distance over which the device can be used accurately. One solution to this problem has been to coat the prism faces with a silver film sealed with a liquid sealer. However, the mirror surfaces formed on the prism faces, while preventing the accumulation of dust thereon, also reduce the reflectivity of the prism.
One general object of the present invention is to provide apparatus for sealing the rear side of a prism, and the like, which overcomes the above-discussed problems known in the prior art.
Another object of the invention is to provide a sealed prism assembly constructed to operate over widely varying temperature and atmospheric pressure conditions.
Yet another object of the invention is to provide such an assembly which is easily constructed and relatively maintenance free.
The prism assembly of the present invention includes a prism having a cylindrical body region which terminates at one of its ends with an annular flange. Encasing the body region is a cup having a wall portion which fits closely and circumferentially thereabout. The cup terminates with a circular end which contacts the prism's flange. An elastomeric sealant distributed between the cup's wall portion and the prism's body region forms an expandable-contractable, air-tight annular seal therebetween.
In a preferred embodiment of the invention, the cup and prism are encased in a canister which is capped at one end. The prism and cup are held in the canister in an axially compressed condition to bias the cup against the prism flange.